The present invention relates to optical devices using thin optical films to adapt light radiation to the conditions of operation. The invention is aimed at showing how to mount and use these thin optical films with the minimum risk of deterioration in their optical properties.
Thin optical films are well known components in optical construction. They are used in a wide variety of optical devices, especially in optical structures such as those using liquid crystal display screens.
These thin optical films act on light radiation by giving it characteristics required for operation. They fulfill various functions: for example, the function of scattering light, modifying the angle along which light radiation is transmitted, polarizing light or attenuating light etc. The thin optical films are therefore used to set up an adapter of light radiation. An adapter of this kind may comprise one or more thin optical films, depending on the corrections to be made to the radiation.
Thin optical films generally take the form of very thin films made of plastic (for example polyester, acrylic etc.), with thicknesses ranging for example from 0.05 mm to 0.5 mm approximately. There are mainly two known ways of mounting these thin optical films in an optical structure to form a radiation adapter: one is called the xe2x80x9cstackingxe2x80x9d method and the other is called the xe2x80x9cadhesionxe2x80x9d method.
FIG. 1 gives a schematic view of an optical structure 1, comprising a radiation adapter aR1 formed by several thin optical films 4a to 4d mounted according to what is called the xe2x80x9cstackingxe2x80x9d method. In the non-restrictive example shown, the four thin optical films 4a to 4d are superimposed and each of them fulfils a different function. They are positioned between a first plate and a second plate 2, 3, made of glass for example. In this type of mounting, the two plates 2, 3, and the thin optical films 4a to 4d form an optical sandwich, whose different elements are kept pressed to each other between walls 5 of a mechanical supporting assembly. It must be noted that, in this assembly, all the elements are fixedly joined to each other in a rigid way. This has the effect, in particular, of generating high mechanical strains on the periphery. During storage at high temperature ( greater than 50xc2x0 centigrade) for example, the thin optical films are held fixed on the periphery and therefore cannot expand as they should. They therefore get deformed, forming corrugated features for example.
FIG. 2 gives a schematic exemplary view of an optical structure 1b in which a radiation adapter aR2 has a single optical film 6 carrying out, for example, a scattering function. The optical film 6 is mounted by the method known as the adhesion method. In this mounting, the scattering film 6 is fixedly joined by one of its faces to a transparent plate 8, made of glass for example to which it is joined by means of a layer 9 made of an adhesive substance. In this second mounting, the elements 6,8 are joined fixedly and less rigidly than in the first mounting, by means of a layer 9 of an adhesive substance. This creates mechanical strains in the film and has other drawbacks related to the presence of the adhesive substance.
In both these types of mounting, the thin optical films remain relatively fragile elements with optical characteristics that can easily deteriorate under the effect of factors in the environment in general, such as for example:
temperature variations which may give rise to thermal expansion phenomena having different amplitudes between the glass plates and the thin optical films, causing mechanical strains and deformation that are harmful to optical quality; this defect is more pronounced in the case of mounting by stacking (FIG. 1) than in the case of mounting by adhesion (FIG. 2); the latter however has another drawback that consists of the major risk of defects due to trapped dust, local variations in the thickness of the adhesive substance, blisters, air bubbles etc.
in both forms of mounting, moisture may substantially modify the optical properties of certain optical films; it must be noted that is practically impossible to clean these films without leaving traces that are themselves a cause of disturbance; moisture has particularly harmful effects and is particularly dangerous because the optical films are sensitive to it even after their mounting:
the optical films may also be subjected to chemical corrosion, which may modify their optical characteristics.
It must also be seen that the mounting by itself of thin optical films is constituted by a sequence of lengthy and delicate operations, especially because of the precautions to be taken in the handling of these films. It can thus happen that films undergo irreversible deterioration because of careless or clumsy handling, when they are mounted in the optical structure, but also during intervention on the other elements of the optical device in which these films are mounted.
The above explanations show, firstly, that the implementation of these thin optical films raises many difficulties by itself and, secondly, that even after their mounting, these optical films remain sensitive to the conditions of their environment, and may have characteristics that vary as a function of this environment. It must be noted that this sensitivity to the environment, in certain fields of application, may have considerable importance as is the case for example for optical devices of the type used in aircraft, helicopters etc.
The present invention is aimed not only at facilitating the mounting of these thin optical films in optical devices but also at making them more independent of environmental conditions. To this end, the invention proposes the sheltering of the thin optical films in an enclosed space built in order to contain them and preserve them from environmental corrosion.
The invention therefore relates to an optical device comprising at least one first optical structure, a light radiation adapter comprising at least one thin optical film, the radiation adapter being exposed to a light radiation that it adapts and transmits to the first optical structure, characterized in that it furthermore comprises an enclosed space having transparent walls and containing the said radiation adapter.